Magnetic return circuit



Alm] 18 1933' E. F. NORTHRUP MAGNETIC RETURN CIRCUIT Filed oct. 22, 195o `3 SheetS-Sheet l April 18, 1933. y E. F' NORTHRUP 1,904,665

MAGNETIC RETURN CIRCUIT `Filed Oct. 22, 1930 3 Sheets-Sheet 2 1 April 18, 933 E' F. NORTHRUP 1,904,665

` MAGNETIC RETURN CIRCUIT `Filed OC.. 22, 1950 3 SheST/S-Sheeb 5 Patented Apr.` 1,8, 1933 UNITED STATES EDWIN FITCH NORTHRUP, OF PRINCETON, `NEIN JERSEY, ASSIGNOR T0 .AJAX ELEC- y PATENT OFFICE TROTHERMIC COBPORAT'ION, 0F .AJAX PARK, NEW JERSEY, A CORPORATION OF N EW JERSEY MAGNETIC RETURN CIRCUIT Application led October 22, 1930. Serial No. 490,514.

My invention relates to methods and ap'- paratus for eliminating stray field about an inductor coil, which may be the inductor of an electric furnace, a reactor orany similar coil.

A purpose of my inventionis to increase the effectiveness of a compensator coil in eliminating stray field about an inductor by reducing the reluctance of the magnetic path between the inductor and the compensator coil, through interposing magnetic material be# tween the inductor and the compensator coil.

A further purpose is to employ a material of high magnetic permeability and low electrical conductivity to cooperate with a compensator coil in eliminating stray field about an inductor. i

A further purpose is to interthread magnetic material of suitable electrical properties between the turns of an inductor coil or ot a compensator coil or of both coils, with or without continuity of the interthreading material from coil to coil.

A further purpose is to employ for such interthreading a magnetic material which, in layers of a thickness not greater than the distance between adjacent turns of a coil, has an electrical break-down voltage higher than the voltage drop between individual turns of the coil.

A A further purpose is to disperse electrically conducting magnetic particles throughout material of low electrical conductivity, making the concentration of magnetic particles so low that the electrical conductivity of the resultant mass is low, while its magnetic permeability is high.

i A `further purpose is to separate magnetic particles for use about an inductor coil by particles of heat insulation or refractory.

A further purpose is to place about an inductor coil an uncompressed nnonhomogenel ous mass of particles, some of which arev magfnetic and electrically conducting and others of which are of high electrical resistance.

. involved.

Figures land 2yare vertical central sections u A further purposeris' firmly to position an inductor coil and a compensator coil with respect to one-another and to a furnace frame f between the two work by packing solidl high electrical re- `to protect the coil from the heat of the furnace and to concurrently eliminate stray field.

A further purpose is to construct one wall of an inductor coil cooling spray chamber of magnetic material and preferably embed a compensator coil within the magnetic mate A further purpose is to reduce the reluctance of the magnetic path between` opposite turns of a focus inductor, or between an in' ductor and a focus inductor, or about an inductor outside of a focus inductor, desirably placing magnetic material of high electrical resistance in physicalv contact with the turns or continuously between the turns, or in both places.

Further purposes will appear in thfefzspecification and in the claims.

My invention relates both to the methods involved and to apparatus by which the meth` ods may be carried out. p

I have preferred to illustrate-.my-invention by a few forms only among' the; many in which it may appear, selecting forms which are practical, efficient, and reliable, and which at the same time well illustrate the principles of diagrammatic furnaces embodying my invention.

Figure 1a is a fragmentary diagrammatic view, showing the magnetic iield a out an individual inductor and compensator turn of Figure 1 where no magnetic material is present.

Figure 1b corresponds to Figure 1a, but shows the field when magnetic material is located between the inductor and the compensator.

Figure 3 is a fragmentary view corresponding to a portion of Fi ure 2, but showing a somewhat diiierent variation. s

Figure 4 is a vertical central section of a slightly different structure embodying my invention.

Figure 5 is a diagrammatic vertical central section of a furnace to which my invention has been applied.

Figure 6 is a modiiied fragment of Figure 5.

Figure 7 is a vertical central section of one formof complete furnace embodying my invention.

Figure 8 is a top plan view of a focus inductor furnace to which my invention has been applied.

-Figure -9 is a section upon the line 9-9 of Fi ure 8.

igure 10-is a modified fragment of Figure- 9.

Figure 11 corresponds generally to Figure 10, but shows the magnetic material differently placed.

In the drawings similar numerals indicate like parts.

In my Patent No. 1,744,983, for inductor furnace, ranted January 28, 1930, I have described t e principles of eliminating stray field by means of a compensator coil placed outside of the inductor andv inductively or directly connected in the circuit in any one of a number of ways there shown.

At any given instant the current carried by the compensator coil will be opposite to that in the ad`acent turns of the inductor, either because t e compensator coil is directly and oppositely connected in the circuit of the inductor coil or because the compensator coil is nductively coupled with respect to the inductor and therefore carries an opposite induced current. l

InFigures 1, 2, 3, 4, and 5, I show a Crucible 2O surrounded b an inductor coil 21, shown in most of the gures as formed of hollow flattened water cooled tubing. It will .be understood that the inductor coil may be of any of the well recognized types, whether solid or hollow, and no matter what the method of cooling and the electrical vconnections.

The inductor coil is surrounded by a compensator coil 22. In Figure 1 the inductor and compensator coils are connected elecidentical with that of Figure 2 except that in Figure 4 the short-circuiting connection 28 for the inductively coupled compensator coil includes a capacity 29.

In Figure 5 the circuit of Figure 1 is diagrammatically illustrated.

The circuits here shown and others not shown are well known in the art of employing compensator coils, and those shown are described merely for completeness.

Considering only the inductor coil 21 and the compensator coil 22 of Figure 1, individual adjacent turns of each of which are shown at 21 and 22 in the diagrammatic view, Figure 1a, the current at a given instant may be assumed to becoming up in the inductor coil turn 21' and passing down in the compensator coil turn'22, as indicated bythe dot and plus signs. yUnder these conditions, the field about the inductor turn 21 will be counterclockwise, while that about the compensator turn 22 will be clockwise.

The directions of the flux inside and outside the inductor turn 21 due to that turn are indicated respectively by the arrows 30 and 31. Similarly, the directions of the flux due to the compensator turn 22 inside and outside of that turn are indicated respectively by the arrows 32 and 33.

T us it will be seen that, under any assumed conditions, the arrows 31 and 32, indicating the aths of flux travel between the inductor an same direction, in icating that the iluxes cooperate in the space between the inductor and compensator colls. Due to the limited range of t e com nsator coil field, since it may be adjustede as to size and location, the amount of flux from the compensator coil which passes inside the inductor coil is negligible, so that the ilux travel indicated by the arrow 30 inside the inductor coil is unoposed.

n' the other hand, flux from the inductor coil which path indicated by the arrow 34, is opposed and diminishedv or eliminated by the flux of the7 compensator coil whose path is indicated by the arrow 33."

Then it will be understood that the compensator coil assists the ileld of the inductor com ensator coils, are in the.

asses outside ofthe le ltimate return circuit of the inductor coil, ta ing the v trical characteristics of the compensator coil.v

The above discussion has merely set forth the well understood principles with regard to compensator coils. I have discovered that great advantage may be gained b interposing magnetic material continuous y or magnetic material of high electrical resistance between the inductor coil and the compensator coil. i

In Figure 1 I illustrate the magnetic material at 35, consisting of a mass having low 'magnetic reluctance and high electrical resistance sol as to avoid the' development of excessive eddy currents. I prefer to form the magnetic mass of magnetic particles disv persed throughout a nonmagnetic medium of high electrical resistance, as more fully explained below.

The magnet-ic material is shown in the various figures as supported by a suitable nonmagnetic shell 36, which takes different forms according to the special needs of surface, and which may be eliminated altogether where the magnetic mass is self-sustaining,

or is supported separately, as for example, by the frame structure of the furnace.

In Figure 1b I illustrate turns 21 and 22 from the inductor and compensator coils on the left hand side of Figure 1, as shown in Figure la. However, differing from Figure 1a, magnetic mate-rial 35 is placed between the inductor and compensator coils.

The distribution of the lines 'of force inside the inductor coil will not be affected by the magneic material, but most of the lines of force which otherwise would take the path indicated by 34, outside of the compensator coil and opposed by` the linesof force indicated by 33, will now follow a path inside instead of outside of the compensator coil.

l A relatively weak fieldrepresented by the arrow 34', will in'some instances tend to pass beyond the4 compensator coil, but will be eliminated completelyand with ease. Thus where, in Figure v1a, many of the lines ofv force go into stray field which the compensator coil is obliged to eliminate, in Fi ure 1b, due to the increased permeability o the produced by a given compensator coil. Sim.-

ilarly, I may obtain stray field elimination, equal to that which occurs when no magnetic material is used, by employing a much small.- er compensator coil, fewer compensator coil turns, or a lower compensator coil current.

In -addition to these advantages in more .essary lto pass a given magnetic field across the return circuit of the inductor coil.

My invention may be applied by placing magnetic material between the turns of the inductorcoil or of the compensator coil or of both, and by providing a complete magnetic path between the compensator and inductor coils.

In Figure 2 the magnetic material 35 eX- tends between the turns of the inductor coil as at 37, thus ,increasing'the permeability of the magnetic path between the turns.

As will be explained below, the magnetic material for this use must of course be of high electrical resistance, as otherwise the turns would be short-circuited. Even if insulating material were placed about thel turns to prevent short-circuiting, excessive eddy currents would build up in the material 35', were it of low resistance. In the form of Figure 2 the magnetic return circuit has very low reluctance, so that most of the lines of forcel will remain in the material 35', and relatively few will need t'o be cut off by the compensator coil.

In Figure 3 they magnetic material 352 is placed between the turns of the compensator coil at 38, but not between the turns of the inductor coil. In this case the inner part of the return circuit Vofthe inductor coilwill take place in nonmagnetic material, but the outer part of the return circuit will take place in magnetic material,l and the bulk of the lines of force will take the return path through the magnetic material. Here also, relatively few vlines of force will pass beyond the magnetic material 352 to be eliminated by the compensator coil.

. lIn Figure 4,the magnetic material 35a `is placed between both the inductor and compensator `coil turns, as at 37 and 3K8 respectively.` The magnetic material 35a provides a complete magnetic path from the inductor coil to the compensator coil, so that none of the return magnetic 'circuit of either the inductor or compensator coil need take place in nonmagnetic material, as would be the casein Figures 1, 2 or 3.

The form of Figure 4, besides being desirable for theoretical reasons, is advantageous luf) a magnetic material 35s, and, if the magnetic.

in construction because both the inductor and compensator coils are firmly positioned by tion is intended to be merely conventional,

since in many cases the furnace structure will consist principally of nonmagnetic materials such as asbestos board, with frame members of steel or of some other metal.

In Figure 5 I illustrate a crucible surrounded b inductor and compensator coils and sup ied with current from a suitable source E. agnetic material 35 has been placed bey tween the compensator and inductor coils, c and heat insulation 40 is located ,around the in more detail.

Crucible inside the inductor coil. The magnetic material itself here also'performs a heat insulating function. This arrangement is desirable because magnetic material should not lbe used inside the inductor coil, as it would cut oi lines of force from the charge.

On the other hand, magnetic-material may desirably be used between the inductor and compensator coils, and this same material here reduces the conduction offheat to the compensator coil. This is later explained In Figure 6 I show a supposed line oi Juncture between the heat insulation 40 and the magnetic material at the inductor coil turns 212. From this figure it is seen that the heat insulation eXtendsypart Way through the inductor coil from the inside, as at 4l, While the magnetic material extends part Way through from the outside as at 42, forming a line of juncture 43 with the heat insulation. Thus the inductor coil is solidly sup` ported and electrically insulated by the materials and 35, and yet its magnetic field will be unaffected inside, but will be distorted inwardly on the outside.

The magnetic material will perform other functions in addition to that of increasing the pe vmeability of the 'magnetic return circuit. In Figure 7 I illustrate a furnace construction in which magnetic high resistance material assiststhe compensator coil-in cutting off stray field from the inductor coil, insulates the compensator coil against excessive heat from the charge and confines the cooling medium in a chamber surrounding the inductor coil.

The furnace of Figure 7 comprises a Crucible 20 surrounded by heat insulation 40 of any suitable material, preferably finely divided refractory tamped in place. A retaining shell 44 of miconite supports the musees refractor The inductor coil 21I is wound from solid stock, and is firmly held by an air dried shell 45 of molded refractory, preferably consisting of a mixture of l70% of sand, 20% ofthermolith (chrome ore with a binder.) and 10% of sodium silicate.

' Between the turns of the inductor I place 4suitable insulating layers 46.

The inductor coil 21s is coated with a water-proofing material which is also desirably an electrical insulator and which will stand a temperature of several hundred'degrees Fahrenheit. I have used a so-called white porcelain which is sprayed on and of which there are several on the market, one under the name o f Insalute. This coating, shown at 47, protects againstl leakage of liquid through the coil and against shortcircuiting of the coil. y

The water-proof coating 47 is lin turn covered by a suitable tape 48 which has a wick action in carrying water or steam to the surface of the inductor coil. I, find asbestosto be a desirable material for the tape 48. f

Outside of the inductor coil I provide a vapor space 49 in which steam may develop when cooling water is vaporized. Water carried through the lcoil 222 is distributed through spray nozzles 50 against the tape 48 and the covered inductor coil. The coil 222 is short-circuited `or otherwise suitably connected in the circuit to serve as a compensator coil, eliminating stray field from the inductor as in the case of a typical compensator coil.

About and chiefly inside of the turns of the compensator coll I place a suitablehigh resistance magnetic material 35, which assists the compensator coil to eliminate stray field as in the forms explained above.

It will be noted particularly thatl the relation of the magnetic material to the inductor and compensator coils in' Figure 7 is similar to that dagrammatically il ustrated inFigure 3, except that magnetic material is placed. outside the compensator coil (as well as within it), thus causing part of the viux outside the vcompensator coil to take a path through. the magnetic material, opposing any stray field from the inductor which may pass beyond the compensator coil.

The magnetic material 35 may desirably contain heat insulating particles between the ma etic particles, so that the compensator coi will be protected from heat developed in the furnace.

As the inductor coil becomes highly heated, Water will vaporize in the chamber 49, absorbing as it does so its latent heat of vaporization from the inductor coil and surrounding parts. The magnetic material will in this case serve as an outer Wall for the chamber 49, with or without .a water-proof coating on its inner surface. Thus it is possible to gain the advantage of having magnetic material between the inductor and compensator coils without sacrificing the space required for the chamber 49 and without covering the inductor coil 218.

Steam and waterl from the chamber 49 are free to escape through the openings 51 and 52. The opening 52 also allows the bus bars 53 and 54 from the opposite ends of the inductor coil to pass to external contacts, one of which is shown against the bracket 55, insulated by the asbestos block 56 from the steel frame parts 57 and 58.

The outer furnace casing 59 of steel or other suitable materia-l is insulated from the frame parts by the asbestos block 60 and the asbestos rings 61, 62, 63 and 63. A brick base 64 supports the entire furnace. Suitable heat insulation 65 has been packed around the top of the cruciblc and around the spout 66.

In Figures 8 and 9 I show the application of my invention to a focus inductor furnace. The use of focus inductors is explained in my patents, No. 1,378,187 for focus inductor furnace, granted May 17, 1921, and No. 1,378,188 for ladle heating by high frequency currents, granted May 17, 1921.

The crucible 20 is surrounded by heat insulation 40 held in place by a shell 67, preferably of miconite. Outside of the shell may be seen the opposite turns 68 and 69 of the focus inductor 70.-

The inductor coil 21* surrounds the focus inductor and is water cooled through connections 71 and 72. Water connections to the focus inductor are shown at 73 and 74.' Current is passed through the inductor coil from leads connected at 75 and 76, and this serves to induce opposite current in the adjacent turns 69 of the focus inductor. `The current in the inner turns 68 of the focus inductor has the same instantaneous direction as that in the inductor.

In furnaces of this type I have previously proposed placing laminated magnetic return circuits at spaced circumferential intervals from one another, extending vertically outside of the inductor coil andainterthreading between the opposite sides of the focus inductor coil at spaced points about the coil. These magnetic return circuits have been incomplete in that they are close to the coils atafew points only, and have not always been convenient on account of the s ace limitations between and around the ocus `inductor and the inductor.

By my present invention I avoid all such laminated magnetic return circuits and use magnetic material of high resistance at4 the points desired in relation to the focus inductor and inductor. Since I will ordinarily employ my magnetic material in finely dividedxforin, it may be placed in any location, no matter what the amount of space available and the shape of the space.

In Figure 9 I show magnetic material 35 between the turns of the focus inductor and extending completely aroundv the furnace except where broken at 77 by the ends of the focus inductor. The magnetic material 355 rests upon the furnace base 78 and is covered by a. refractory ring 79.

I also lace magnetic material 35 around `the outsi e of the inductor to reduce the reluctance of the magnetic return circuit of the inductor.

Thus it will be seen that in Figure 9 I provide a path of low reluctance between opposite sides of the focus inductor and also a path of low reluctance for the return circuit of the inductor. It will be evident, however, that magnetic material may be arranged in other ways about a focus inductor furnace.

In Figure 10 ,I show magnetic material 35 between the inductor 21 and the outer turns 69 of the focus inductor. In this case a path of low reluctance exists between the inductorv resistance particles be employed for a magnetic return circuit.

I haverecognized that the electrical resistance of such material may be desirabl increased by reducing the concentration o low resistance magnetic particles in a mass-consisting partially of nonmagnetic particles of high electrical resistance, so that the frequency of contact between the electrically conducting particles will be reduced.

In the past, when such magnetic particles have been employed, the mass has been compressed. This I have discovered to be undesirable, since it places the electrically conducing particles in close contact and greatly reduces the electrical resistance of the mass. I will therefore ordinarily avoid compreing the mass, and in any case I will make the concentration of electrically conducting particles low enough with respect to the compression so that the resultant mass will have a high electrical resistance.

To increase the electrical resistance of the mass, I increase the percentage of particles of high electrical resistance, thusgspacing the electrically conducting magnetic articles from one another by particles of igh resis]t)ance.d th f hi h e en in u on epurpose orw c my magnltic mteiial will be used, vary the character lof the noiimagnetic particles. For exam le, where I desire the resultant mass to be a airly ood conductor of heat (although a vpoor con uctor of electricity), I use high resistance particles of ood heat conductivity. On the other hand, w ere I desire the mass to have heat insulating properties, I use high resistance particles of poor heat conductivity, as for example refractory particles such as zircon.

The characteristics of the magnetic material from an electrical standpoint will of course vary with the strength o he magnetic field, with the stren h of the electric field at the point where t e magnetic material is to be placed, and with the frequency of the electrical current. I preferably operate at a point upon the magnetic saturatipn curve for my magnetic material below complete saturation, in order to obtain the maximum permeability consistent with high electrical resistance. Thus Iv increase the concentration of magnetic particles as much as possible without decreasing the electrical resistance below the amount permissible.

Where the current in the` inductor is of high frequency, I require high electrical resistance in'my magnetic material as a composite in order to avoid the develbpment of excessive eddy currents. Because, b regulating the ma etic particle size, an average distance o separation I may increase at wlll the electrical resistance of the ma etic mass, my invention is particularly ap hcable to high frequencies, where the nee of reducing eddy currents is much greater than at lower frequencies. v

The magnetic particleswill ordinarily be iron filings or similar particles. They may be of course particles of very high permeability, as for example transformer iron or hi h silicon steel.

ere the mass need not have heat insulating properties, the high resistance particles may consist of zinc oxide.

The relationship between the size of the low reluctance particles and that of, the high resistance particles in the mass `is important. If one sort of particles is considerably larger than the other sort, the smaller particles will fit into the interstices of the larger particles, and direct particle contact from one low reluctance (low resistance) particle to another will not be eliminated. Then, under this condition, the electrical resistance of the mass will be correspondingly low. If the high resistance particles are alsoheat insulators, the mass nevertheless will be a poor heat insulator.

However, if both sorts of particles are of comparable size, the high resistance particles will space the low reluctance (low resistance) particles from one another, and the mass will have high resistance. Also, if the high resistance particles are good heat insulators, the mass will be a goed heat insulator in that cas. C

f I therefore will preferably make all particles of the non-homogeneous'magnetic mass of the same size. Y

It is not necessary to useparticles of high electrical resistance at all in order to obtain a magnetic mass of high electrical resistance. The magnetic particles themselves may bc coated with a high resistance film, described by me as an illustration but not claimed as a species by me, in order to increase the contact vresistance from particle to particle, thus increasing the electrical resistance of the mass. T his has the advantage that the magnetic permeability will be higher than in a magnetic mass containing nonmagnetic high resistance particles, because permeability depends upon nearncss or separation of thc magnetic particles rather than upon clcanness of contact between them.

The high resistance coating for the surface of the magnetic particles may be a layer of iron oxide formed on the surface, or a passive outer layer obtained by treating the iron particles with cold concentrated nitric acid.

However, in its best form it consists of separate insulating material applied to thc surfaces of the particles. A satisfactory material is porcelain enamel, of which there are several on the market, one being known as"Insalute. Insalutc has the advantage of resisting a considerable rise in temperature, a condition inevitable in a magnetic material which must be placed close to a hot charge. The magnetic mass may also have heat insulating properties secured by a particle coating which is a heat insulator. Insalute is again advantageous for this purpose.

In view of my invention and disclosure va.- ri'ations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain part or all of the benefits of my invention without copying the structure shown, and I, therefore.` claim all such in so far as they fall Within the reasonable spirit and scope of my invention.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is l. In a stray field eliminator for inductor coils, a compensator coil surrounding the inductor coil and instantaneously carrying current in the opposite direction from that of the current in the inductor coil and finely divided magnetic particles forming a mass between the compensator and inductor coils occupying continuously a considerable circumferential extent about the inductor.

2. In a stray field eliminator for inductor coils, a, compensator coil surrounding the inductor coil and instantaneously carrylng current in the opposite direction from that of the current in the inductor coil and magnetic material continuously annularly surroundlll lzo

ing the inductor coil between the compensator and inductor coils.

3. In a stray field' eliminator for inductor coils, a compensator coil surrounding the inductor coil and instantaneously carrying current in theoppositc direction from that of the current in the inductor coil and finely diyided magnetic particles annularly surrounding the inductor coil between the compensator and inductor coils. i

4. .In a stray vfield eliminator for inductor coils, a compensator coil surrounding the inducton coil and instantaneously carrying current in the opposite direction from that of the current in the inductor coil and magnetic material of high electrical resistance between the compensator and inductor coils.

5. In a stray field eliminator for inductor coils, a compensator coil surrounding the inductor coil and. instantaneously carrying current in the opposite direction fromthat of the current in the inductor coil and finely divided'magnctic material of high electrical resistance between the compensator and inductor coils. 4

6. In a stray field eliminator for inductor coils, a compensator coil surrounding the in'- ductor coil and instantaneously carrying current in the opposite direction from that of the current in the inductor coil and magnetic i material of high electrical resistance annularly surrounding the inductor coil'between the compensator and inductor coils.

7 In an electric induction furnace, an inductor coll, a compensator coil surroundmg.

the inductor and adapted to carry current instantaneously in an opposite direction from that of the current carried by the inductor coil and magnetic material annularly surrounding the inductor coil between the compensator and inductor coils.

8. In an electric induction furnace, an inductor coil, a compensator coil adapted to carry. current instantaneously in an opposite direction from that of the current carried by the inductor coil and finely divided magnetic particles between the compensator and inductor coils.

9. In' an electric induction furnace, an inductor coil, a compensator coil surrounding the inductor and adapted to carry current instantaneously in an opposite direction from that of the current carried by the inductor coil and a. non-homogeneous mass of magnetic particles mixed with high resistance particles between the compensator and inductor coils.

10. In an electric induction furnace, an inductor coil, a compensator coil surrounding the inductor and adapted to carry current instantaneously in an opposite direction from that of the current carried by the inductor coil andv magnetic material between the turns of the inductor coil and outsideof the inductor coil between the compensator and inductor coils.

11. In an electric induction furnace, an inductor coil, a compensator coil surrounding the inductor and adapted to carry current instantaneously in an opposite direction from that of the current carried by the inductor coil and magnetic materia-l ofV high electrical resistance between the turns of the inductor coil and annularly Isurrounding the inductor coil between the compensator and inductor coils.

12. In an electric induction furnace, an inductor coil, a compensator coil. surroundi-ng the inductor and adapted to carry current instantaneously in an opposite direction from that of the current carried by the inductor coil and a magnetic core inserted between -the turns of the compensator coil.

13. In an electricinduction furnace, an inductor coil, a compensator coil surrounding the inductor and adapted to carry current instantaneously in an opposite direction from that of the current carried by the inductorcoil and a magnetic core between the turns ,of the compensator coil and within the compensator coil.

' 14. yIn an electric induction furnace, an inductor coil, a compensator coil surrounding the inductor and adapted to carry current instantaneouslyl in an opposite direction from that of the current carried by the inductor coil and magnetic material of high electrical resistance between the turns of the compensator coil.

15. In an electric,v induction furnace, an inductor coil, a compensator coil surrounding the inductor and adapted to carry current instantaneously in an opposite direction from that of. the current carried by the inductor coil and magnetic material of high velectrical resistance between the turns of the rent instantaneously in an opposite direction from that of tli/e current carried by the inductor coil a gl magnetic material between the turns of tie inductor and compensator coils and annularly between the inductor and compensator coils.

17. In an electric induction furnace, an inductor coil` a compensator coil surrounding the inductor and adapted to carry current instantaneously in an opposite direction from that of the current carried by the inductor coil and magnetic material of high electrical resistance between the turns of the inductor and compensator coils and annularly between the inductor and compensator coils. y

18. In an electric induction furnace, an iuductor coil, a compensator coil surrounding llO the inductor and adapted to carry currentv ductor coil and magnetic material extending annularly around the outside of the compensator coil.-

1$ In an electric induction furnace, an inductor coil, a compensator coil surrounding vthe inductor and adapted to carry current instantaneously in an opposite direction fromthat ofthe current in the inductor',

- i0' heat, insulation 'within the inductor and against its turns on the inside and magnetic material outside of the-inductor and against itsturns on the outside,-whereby the reluctance' of the magnetic path is unchanged on the inside, but is decreased on the outside of the inductor to assist the compensator in reducing stray field.

20.1 11-fan electric induction furnace, an inductbrrfcoil, a com ensator coil surrounding the inductor an adapted to carry curl rent instantaneously in an opposite direction from that of the current' in the inductor, heat insulation within the inductor and be. tween its' turnstoward the inside and magnetic materialfoutside of the inductor and suicient distance to permit application to the inductor of the cooling medlum, whereby the cooling medium 1s externally confined by the magnetic material and the stray field from the inductor is short-circui through it.

22. In an electric induction fu mace, an

inductor coil externally exposed for heat transfer, a compensator coil surrounding the inductor and instantaneously adapted to carry current in theo posite direction from that of the current in tlie inductor, spray noz- 'zles,y for application of a cooling medium in heat transferring relation to the outside of Athe inductor and magnetic materialwithin the compensator coil annularly surrounding andspaced from the induotcra suicient distancepto permit application to the inductor1 of a cooling medium.

'magnetic material of resistance individual turns of the ocus inductor andV axial outer and inner, focus inductor coils and finely divided magnetic particles between the focus inductor coils for improving the iux path.

' 25. In an electric induction furnace, an inductor coil, a focus inductor within the inductor coil having o posite portions of individual turns radiali; spaced from one another and magnetic material between the opposite sides of the individual focus inductor turns conforming to the path of those turns.

26. `In an electric induction furnace, an inductor coil, a focus inductor within the inductor coil having o posite portions of individual turnsV radial y spaced from one another and magnetic material of high electrical resistance between the opposite sides of the individual focus inductor turns and following the course of the turns.

27. In an electric induction furnace, an inductor coil, a focus inductor within the inductor having op 'te portions of individual turns radi ly eed from one another and finely divldgd. magnetic particles between the turns of the focus inductor.

28. In an electric induction furna, an inductor coil, a focus inductor within the inductorv havin o te rtions of individual turns adimys'lspa from one another and a heat insulating mass containing h h electrical resistance annularly surrouriing the focus inductor.

29. In an electric induction furnace, an inuctor clpil, a focus inductor within fthe iinuctor ving' o 'te rtions o 'in ividual turns from one anetic material of high electrical tween they op' te sides of the other, m y

following the course of these turns and magnetic material surrounding' the inductor.

30. In an electric induction furnace, an inductoncoil, a focus inductor within the inductor coil having o posite portions of individual turns radial y spaced from one another and magnetic material between the inductor and the outside of the focus inductor annularl Vsurroun the focus inductor.

DWIN FI CH NORTHRUP.

23. In an electric induction furnace, an inductor coil externally exposed forL heat transfer,' a/ hollow water-carrying fcompensator coil surrounding the inductor, spray nozzles communicating with the interior of the com- 6,0 pensator coil at intervals along its length and directing water in heat transferring relation to the outside of the inductor and magnetic material of high electrical resistance about the compensator coil outside of the spray nozzles.

. 24.",In an electric induction furnace, co- 

